Methods of making magnetic cores



1958 J. A. GESHNER 2,847,333

METHODS OF MAKING MAGNETIC CORES Filed July 10, 1956 2 Sheets-Sheet 1 INDIVIDUAL COATINGS -OF' ORGANIC FINISH MAGNETIC CORE FIG. 2

INVENTOR. BY J. A. GESHNER United States Patent "Ofifice 2,847,333 Patented Aug. 12, 1958 2,847,333 METHODS or MAKING MAGNETIC CORES John "Allen 'Ge'shner, Downers Grove, Ill., a'ssignor to Western Electric (lompany, Incorporated, New York, N. Y., a corporation of New York Application July 10, 1956, Serial No. 596,866

8 Claims. ((51. 117- 232) This invention relates to methods of making magnetic cores, and in particular to methods of 'making stabilized -m'ag'r'ietic' cores of the compressed dust type.

One type of inductive loading coil used extensively in the communications field includes a generally toroldalshaped magnefiecore and a wire winding thereon. The

magnetic core comprises finely divided particles of a magnetic material. The particles are coated individually with a combination insulator and binder material, and then are compressed into a core body of the desired shape. Before such magnetic cores are provided with their'wire windings, a protective coating may be applied to the surfaces of the magnetic cores, which is designed to provide a high degree of both mechanical and electrical protection.

'In the manufacture of loading coils of this type, it is desirable to control the temperature stability of inductance of the finished coils. A conventional test for the temperature stability of inductance involves placing a sample coil in a desiccator and subjecting the coil to various temperatures for prolonged periods of time, while simultaneously taking inductance measurements at periodic intervals. The inductance er the sample coil with varying temperatures and time is observed, and

'the percentage change'in the inductance from'zero to a IP16 coil are-recorded throughout the test and compared with the measuredvalueof inductance of thesample coil before the start of the test, whereby there is obtained the stability coefiicierit 'of the particular batch from which the sample coil was'taken. Although it is desirable that the stability coeflicient be zero, finished loading coilsh'a'ving stability-coefiicients within the range of :0.1% are acceptable in the manufacture of loading coils having molybdenum-permalloy powder magnetic cores.

The stability coefiicient of uncoated coils is found generally to be ofthe orderof from about +05% 'to about Toreduce the stability coefiicient to within the desired tolerance obi-0.1%, it has been the practice to add to the molybdenum permalloy dust from which the magnetic cores are formed a sufiicient-quantity of a low Curie point stabilizer dust, having a negative stability coefficient, to reduce the stability coeflicient of the finished loading coils to within the desired range. It would be advantageous if the desired stability coefiicient of a finished loading c'oil could b'eobtained in some other manner.

It is'an object of this invention to provide new and improved methods of maltiiig magnetic cores.

Another object of this invention is to provide new and improved methods of making stabilized magnetic cores of the compressed dust type.

A method of making magnetic cores illustratingcertain features of the invention may include the steps of forming such a core from a magnetic material having characteristics such that the formed core has a positive temperature stability of inductance coefficient, and covering the magnetic core with at least one coating of a material having a negative stability efiect sufiicient'to reduce the positive temperature stability of inductance coemcient of the core.

A complete understanding of the invention may'be obtained from the following detailed description of'methods forming specific embodiments thereof, when read in conjunction with appended drawings, in which:

Fig. 1 illustrates a magnetic core of the compressed dust type having its electrical characteristics stabilized in accordance with the present invention;

Fig. 2 is an enlarged, fragmentary vertical section of the magnetic core shown in Fig. 1;

Fig. 3 is a graph illustrating thepercentage inductance variation with time and temperature for a typical loading coil having an uncoated magnetic core;

Fig. 4 is a graph illustrating the percentage inductance variation with time and temperature fora loading; coil with a magnetic core formed from the same batch of alloy dust as the last-mentioned magnetic core, but having's'everal layers of an organic finish, and

Fig. 5 is a graph illustrating the percentage inductance variation With time and temperature for a loading coil with a magnetic core formed from the above-mentioned batch of alloy dust, but'having a greater predetermined number of layers of the organic finish. i

The present invention is designed primarily for the stabilization of electrical properties of molybdenumpermalloy dust magnetic cores. In carrying out thepresent invention a large number of magnetic cores are fabricated by a conventional process from a common batch of insulated "alloy dust which is placed in molds and compressed into generally toroidal-shaped'magnetic cores of identical dimensions. A sample magnetic core is selected at random and provided with a wire winding of a predetermined number of turns. The wound magnetic core is then placed in a desiccator and subjected to a temperature stability of inductance test of the type described hereinabove. During this test inductance measurements are made periodically to determine the stability coefiicient of the magnetic cores WlthOlltfl protective surface coating.

Illustrated in Fig. 3 is agraph'of the results ofsuch a temperature stability test performed on a typical magnetic core without a surface coating. From Fig. 3 it may be seen that the sample magnetic core so tested has 'a stability coefficient of approximately +0.57%, which is not within the allowable tolerance of 10.1%. This value of +0.57% may be taken as the stability coeflicient for the entire group of magneticcores fabricated'frorn the particular common batch of insulated alloy dust.

In accordance with this invention, thepositivestability coei-ficient of this particular group of magnetic cores may be reduced to within the desired tolerance of :O.l% by applying successive coatings of one or more pigmented 3 ing may be accomplished by an initial air drying period followed by a period of baking at an elevated temperature of the order of 350 F. The air drying period should be sufiiciently long to prevent blistering during the subsequent baking operation. It is desirable that each of the individual coatings of finish so formed be impervious and have a minimum thickness of approximately .0004 inch.

It has been found that coatings of certain organic finishes have a negative stability coefiicient effect and, that, by applying a predetermined number of successive coatings of such organic finishes to a magnetic core fabricated from a given batch of insulated alloy dust, at finished coil having a stability coefiicient within the acceptable tolerance of i0.l% may be produced. Successive coatings of such organic finishes have an accumulative effect, that is, the predetermined negative stability coefficient effect of one coating adds to the predetermined negative stability coefficient effect of other coatings to produce a greater negative stability coefficient effect.

To determine the number of layers of a given organic finish or finishes required to achieve a stability coefiicient of i0.1% for a particular group of coils having magnetic cores fabricated from the same batch of insulated alloy dust, sample magnetic cores are tested in accordance with the previously described test procedures for temperature stability of inductance. Once the number of layers of such organic finishes required to produce a finished coil having a stability coefficient of 10.1% is determined, all of the other magnetic cores are provided with an equal number of coatings in the same manner to achieve similar results.

The following examples illustrate the invention. These examples are given only for the purpose of illustration and are not intended to limit the invention.

Example I Percent By Material Weight of the Varnish Heat reactive, oil soluble varnish resin (butyl phenol formaldehyde, alkaline catalyzed) 47 Industrial xylene 53 A varnish resin of the type described above may be obtained commercially as BR-10282 alkali refined linseed oil varnish manufactured by Bakelite Company, New York city, N. Y.

Each coating of the varnish was sprayed on the core, was suitably air dryed and then baked by infra-red heating at a temperature of approximately 350 F. Each coating of the varnish had a minimum thickness of approximately 0.0004 and a maximum thickness of approximately 0.0007 inch, and Was substantially free from blisters or like imperfections.

The sample magnetic core having three coatings of the varnish was tested for its temperature stability of inductance in the manner previously described. The results of this test are illustrated graphically in Fig. 4. It may be seen from Fig. 4 that three successive coatings of the varnish had a negative stability effect sufiicient to reduce the stability coefficient to +0.17%. However, a sta- 4 bility coefficient of +0.17% is still not within the acceptable tolerance of -0.l%.

Another magnetic core was selected from the same group of unfinished magnetic cores and eight successive coatings of the same insulating varnish were applied in the manner hereinabove described. The magnetic core having eight coatings of the varnish finish, when subjected to the temperature stability of inductance test, was found to have a stability coefiicient of +0.1%. The results of the latter test are illustrated graphically in Fig. 5. Accordingly, all of the remaining magnetic cores fabricated from the common batch of insulated alloy can be treated in a like manner (i. e. provided with a minimum of eight coatings of the varnish finish) to produce finished loading coils having stability coefficients within the acceptable tolerance of i0.l%.

The application of additional coatings of the varnish finish further decreases the stability coefiicient and, if a sufiicient number of coatings of the varnish finish are applied, the stability coeflicient will become negative and may even fall without the negative stability coefficient limit of 0.l%. For example, ,the application of twenty coatings of the varnish finish produced a coil having a stability coefficient of 0.19%.

Example 11 Percent by Material Weight of the Varnish Heat reactive, oil soluble varnish resin (butyl phenol formaldehyde, alkaline catalyzed) 32 Industrial xylene 36 Titanium dioxide 32 A varnish resin of the type described above may be obtained commercially as BR-l0282 alkali refined linseed oil varnish manufactured by Bakelite Company, New York city, N. Y.

The individual coatings of unpigmented insulating varnish and pigmented insulating varnish were applied in the same manner described in connection with Example I. The magnetic core having four coatings of unpigmented insulating varnish and one layer of pigmented insulating varnish was tested for temperature stability of inductance and was found to have a stability coefficient of +0.05%. Thus it may be seen that the layer of pigmented insulating varnish has a somewhat greater negative stability effect than a coating of the unpigmented insulating varnish.

Example III As further evidence of this phenomenon, a magnetic core initially having a stability coefficient of +0.57% was provided with two coatings of the unpigmented insulating varnish described previously in Example I. Three coatings of the pigmented insulating varnish described in Example II were then applied, and the resulting coated core was found to possess a stability coefficient of 0.l9%.

It will be understood that this invention is not limited to the specific organic finishes mentioned, namely unpigmented insulating varnish and pigmented insulating varnish. Other suitable pigmented or unpigmented organic finishes having negative stability characteristics include enamels, such as a white baking enamel having the following composition:

Percent b? Material Weight 0 the Enamel Titanium dioxide n 30. 9 Dehydrated castor oil or linseed oil modified glyceryl Ehthalate alkyd resin containing 38 to 45% phthalie anydride 30. 4 Industrial xylene 30. 4 Aromatic solvent (petroleum or coal tar; distillation range 150 to 250C) 8. 3

Further, it will be understood that this invention is not limited'to any specific number of coatings of such material nor is it intended to limit the thickness of such coatings to any specific dimension.

It is manifest that principles of the invention have manifold applications and that the invention is not limited to the specific examples described hereinabove.

What is claimed is:

1. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having characteristics such that the formed core has a positive temperature stability of inductance coefficient, and covering the core with at least one coating of a material having a negative stability characteristic sufficient to reduce the positive temperature stability of inductance coefficient of the core.

2. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having a positive temperature stability of inductance coeificient, and applying a plurality of coatings of a material having a negative stability characteristic to reduce the positive temperature stability of inductance coeflicient of the core.

3. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having a positive temperature stability of inductance coeflicient, and coating the core with a material having a negative stability characteristic until the thus coated core has a substantially zero temperature stability of inductance coeflicient.

4. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having a positive temperature stability of inductance coelficient, and applying coatings of a material having a negative stability characteristic until the thus coated core has a substantially zero temperature stability of inductance coeflicient.

5. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having a positive temperature stability of inductance coefiicient, and applying to the surface of the core a plurality of layers of an organic finish having a negative stability characteristic until the finished coated .core has a temperature stability of inductance coefiicient substantially less than that of the uncoated magnetic core.

6. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having a positive temperature stability of inductance coefiicient, and applying to the surface of the core a number of layers of a varnish having a negative stability characteristic, the number of layers being suiiicient to reduce the temperature stability of inductance coeflicient of the coated core to substantially less than that of the uncoated magnetic core.

7. The method of making temperature stabilized magnetic cores, which comprises forming a core from a magnetic material having a positive temperature stability of inductance coetficient, and applying to the surface of the core a plurality of layers of an enamel having a negative stability characteristic, the number of layers being suflicient to produce in the finished coated core a substantially zero temperature stability of inductance coeflicient.

8. The method of making temperature stabilized magnetic cores of the compressed dust type, which comprises forming a core from a dust-like magnetic material having a positive temperature stability of inductance coeflicient, and applying to the surface of the core a suliicient number of layers of pigmented and unpigmented varnish having a negative stability efiect to produce in the finished coated core a temperature stability of inductance coefficient substantially less than that of the uncoated magnetic core.

References Cited in the file of this patent UNITED STATES PATENTS 434,164 Terry Aug. 12, 1890 2,059,441 Converse Nov. 3, 1937 2,076,230 Gillis Apr. 6, 1937 2,307,588 Jackson et a1. June 5, 1943 

1. THE METHOD OF MAKING TEMPERATURE STABILIZED MAGNETIC CORES, WHICH COMPRISES FORMING A CORE FROM A MAGNETIC MATERIAL HAVING CHARACTERISTICS SUCH THAT THE FORMED CORE HAS A POSITIVE TEMPERATURE STABILITY OF INDUCTANCE COEFFICIENT, AND COVERING THE CORE WITH AT LEAST ONE COATING OF A MATERIAL HAVING A NEGATIVE STABILITY CHARACTERISTIC SUFFICIENT TO REDUCE THE POSITIVE TEMPERATURE STABILITY OF INDUCTANCE COEFFICIENT OF THE CORE. 